1. Field of the Invention
This invention relates to semiconductor light emitting devices, particularly, semiconductor lasers. This invention can be desirably used in a case that high output and long lifetime are demanded as for excitation light sources for optical fiber amplifiers, light sources for optical information processing, and the like. This invention is also applicable to LEDs (light emitting diodes) such as super-luminescent diodes or the like in which optical output comes from their facet, vertical cavity surface emitting lasers, and the like.
2. Description of Related Art
Optical information processing technologies and optical communication technologies have remarkably progressed these days. For example, high density recording by means of magneto-optical discs, two-way communications using optical fiber networks, and the like can be exemplified but too numerous to be counted.
In the field of communication technologies, for example, active research is carried out in various quarters with respect to optical fiber transmission lines of a large capacity sufficiently coping with the upcoming multimedia era, and Er3+-doped optical fiber amplifiers (EDFA) in which rare earth elements such as Er3+ are doped, as amplifiers for signal amplification with flexibility to that transmission method. Under these circumstances, developments of highly efficient semiconductor lasers for excitation light source, as a essential component for EDFA, have been sought.
The emission wavelength of the excitation light source applicable to EDFA theoretically includes three types, 800 nm, 980 nm, and 1480 nm. Excitation at 980 nm, among those, has been known as most desirable in terms of characteristics of amplifiers in consideration of gain, noise figure, and the like. The laser having such an emission wavelength of 980 nm is expected to meet contradicting demands as having a high output as the excitation light source and having a long lifetime. With respect to a wavelength around that wavelength, e.g., 890 nm to 1150 nm, while such a laser is demanded for SHG (secondary harmonic generation) light sources and heat sources for laser printers, other applications as well, developments of lasers with high output and high reliability have been expected.
In the field of information processing, semiconductor lasers are made with higher output and shorter wavelength for high density recording and high speed writing and reading. A higher output is strongly expected for conventional laser diodes (hereinafter referred to as xe2x80x9cLD""sxe2x80x9d) having emission wavelength of 780 nm, and the LD""s having a 630 to 680 nm band have been energetically developed in various quarters.
As semiconductor lasers of about 980 nm, a semiconductor laser durable for continuous use for approximately two years with an optical output around 50 to 100 mW already has been developed. However, in operation under a higher optical output, rapid degradation occurs, and such a laser cannot have adequate reliability. This is the same to the LD""s having 780 nm band and 630 to 680 nm band, and guaranteeing reliability at high output is a major concern for all semiconductor lasers particularly using a GaAs substrate.
One of the reasons of the poor reliability is degradation at a light emission facet of laser beam subjecting to very high optical density. As well known even in GaAs/AlGaAs based semiconductor lasers, many surface states exist at and around the facet, and those serve as nonradiative recombination centers and absorb the laser beam, so that generally the temperature around the facet is made higher than that of the bulk of the laser device, and so that this increased temperature further narrows band gaps around the facet and makes the laser beam more readily absorbed, as explained as occurrence of a positive feedback. This phenomenon is known as a facet degradation or namely COD (Catastrophic Optical Damage) observed when a large current flows at a moment, and sudden failure in devices according to a lowered COD level where a term aging test is made are common problems in many semiconductor laser devices.
Various proposals have been made to solve those problems. For example, various proposes had been made so far to make transparent the band gap at the active layer region around the facet with respect to the emission wavelength to suppress optical absorption around the facet. Lasers having such a structure are generally called window structure lasers or NAM (Non Absorbing Mirror) structure lasers, and these lasers are very effective where a high output is required. In a method to epitaxially grow a semiconductor material transparent to the emission wavelength on the laser facet, however, a subsequent electrode step becomes very complicated because the epitaxial growth is made where the laser is in a so-called bar state.
Meanwhile, various methods to make the active layer disordered by intentionally doing thermal diffusion or ion implantation of Zn, Si and so on as impurities doped into the active layer around the facet of the laser have been proposed (Japanese Unexamined Patent Publication Heisei No. 2-45,992, Japanese Unexamined Patent Publication Heisei No. 3-101,183, Japanese Unexamined Patent Publication Heisei No. 6-302,906). However, the impurities generally diffuse in the LD fabrication process from the epitaxial direction of the laser device to the substrate direction, so that a stable fabrication is difficult because the diffusion depth and lateral diffusion with respect to the direction of the cavity are not easily controlled. In cases of ion implantations, since high energy ions are introduced from facets, damages tend to remain at the LD facets even where anneal processing is made. Moreover, there arises another problem that an increased reactive current accompanying with decreased resistance in the impurity introduced region increases the threshold current and operation current of the laser.
On the other hand, Japanese Unexamined Patent Publication Heisei No. 3-101,183 discloses a method for forming a contamination-free facet and then forming a passivation layer or a part of this layer with the use of an oxygen excluding material undergoing neither any reaction with the semiconductor facet or diffusion by itself.
Generally, operation in air, e.g., in a clean room, cannot prevent nonradiative recombination centers, such as Gaxe2x80x94O, Asxe2x80x94O, generated at the facet during cleaving, from forming. Accordingly, to form the contamination-free facet as described in this Journal, it is necessary to form a passivation layer at the same time as cleavage, but this can be carried out only in vacuum. Cleavage in vacuum requires extremely complicated apparatuses and operations in comparison with an ordinary cleavage done in air. Although this journal describes a method in which a facet is made by a dry etching, such a dry etching is not suitable for a fabrication method of LD requiring a longer lifetime because more nonradiative recombination centers are created in comparison with the facet formed by cleavage.
As similar to this journal, in L. W. Tu et al., xe2x80x9cIn-Vacuum cleaving and coating of semiconductor laser facets using siliconxe2x80x9d and a dielectric, J. Appl. Phys. 80(11) Dec. 1, 1996, described is that where a Si/AlOx structure is cleaved in vacuum in the step of coating onto a laser facet, the carrier recombination speed is retarded, and thus the initial COD level is increased. This article, however, refers to neither reliability over a long time nor the relationship between the coating and the LD structure.
Further, there has been known a technique for inserting an Si layer having an optical thickness corresponding to xc2xc of the emission wavelength at the boundary between the coating film and the semiconductor material as to displace the facet from the anti-node of the standing wave existing in the direction of the cavity, thus lowering the electric field strength at the beam emission facet. However, in a wavelength band that general semiconductor lasers are realized, particularly, 400 to 1600 nm, expected to be high output LD, Si by itself acts as an optical absorber, and thus, the device degradation may be accelerated due to increased temperature at the facet.
Thus, any semiconductor light emitting device and any fabrication method for semiconductor light emitting device, proposed previously, were not satisfactory from a technological viewpoint.
This invention has an object to solve the above problems in the prior art.
More specifically, it is an object of the invention to provide a high performance semiconductor light emitting device capable of stably suppressing the surface state density otherwise externally sensitive on a facet for a long time and establishing both of a high output and a long lifetime. It is another object of the invention to provide a semiconductor light emitting device capable of suppressing degradation at the facets and being manufactured in a simpler method allowing cleavage in air.
The inventors have discovered, as results of diligent researches to accomplish the above objects, an oxidizing state of a portion adjacent to a facet of a compound semiconductor layer or a facet of a passivation layer affects a long term stability of a surface state density at the facet and completed this invention.
That is, this invention is to provide a compound semiconductor light emitting device comprising: a substrate; a compound semiconductor layer containing a first conductive type clad layer, an active layer, and a second conductive type clad layer, the compound semiconductor layer being formed on the substrate; and a cavity structure formed of two opposing facets of the compound semiconductor layer, wherein surfaces of the first conductive type clad layer, the active layer, and the second conductive type clad layer forming the facet of the compound semiconductor are covered with a passivation layer, wherein at least one element constituting the facet of the compound semiconductor layer is not coupled to oxygen, and wherein a portion of the passivation layer adjacent to the facet of the compound semiconductor layer contains oxygen as a structural element.
In the compound semiconductor light emitting device according to the invention, the passivation layer preferably has a portion excluding any oxygen as a structural element, and may exclude any element belonging to 3 through 5 groups. The passivation layer preferably includes one or more elements selected from a group of Si, Ge, S, Se, and Al, particularly, Si.
According to an embodiment, with respect to a passivation layer, exemplified are a structure that portion, in the passivation layer, adjacent to a facet of the compound semiconductor layer is made of SiOx, and the other portion, in the passivation layer, is made of Si, or a structure that a portion, in the passivation layer, adjacent to a facet of the compound semiconductor layer is made of SiOx, and an intermediate portion, in the passivation layer, is made of Si, and a portion, in the passivation layer, adjacent to a coating layer is made of SiOx, or a structure that a portion, in the passivation layer, adjacent to a facet of the compound semiconductor layer is made of SiOx, and an intermediate portion, in the passivation layer, is made of Si, and a portion, in the passivation layer, adjacent to a coating layer is made of SiNx.
The passivation layer preferably has a thickness Tp (nm) satisfying the following formula (I):
0.2 (nm) less than Tp (nm) less than xcex/8n (nm)xe2x80x83xe2x80x83(I)
where xcex denotes an emission wavelength (nm) of the compound semiconductor light emitting device, and n denotes the real part of an average refractive index of the whole passivation layer at the wavelength xcex.
In the compound semiconductor light emitting device according to the invention, the surface of the passivation layer is preferably covered with coating layers made of dielectric or a combination of dielectric and semiconductor. The coating layers preferably include one or more compounds selected from a group of AlOx, TiOx, SiOx, SiN, Si, and ZnS, and are constituted by a coating layer of a low reflectance including at one end, e.g., AlOx, and coating layers of a high reflectance at the other end including, e.g., AlOx and Si. A portion of the passivation layer adjacent to the coating layers may preferably include oxygen as a structural element.
The facets of the compound semiconductor layer are preferably (110) plane and or crystallographically equivalent planes, and preferably, exposed to plasma irradiation before formation of the passivation layer. The surface of the passivation layer may also preferably be exposed to plasma irradiation before formation of the coating layers. Such plasma irradiation desirably includes ion irradiation at energy between 25 eV and 300 eV.
An active layer preferably includes In as a structural element such as Inx Ga1xe2x88x92xAs (0 less than xxe2x89xa61) or (AlxGa1xe2x88x92x)yIn1xe2x88x92yP (0xe2x89xa6xxe2x89xa61, 0xe2x89xa6y less than 1) or the like, and desirably has a quantum well structure.
Moreover, an absolute value of enthalpy of formation of an oxide of an element constituting the passivation layer is desirably greater than an absolute value of enthalpy of formation of an oxide of at least one element constituting a facet of the compound semiconductor layer.
Oxygen contained as a structural element in the passivation layer is the oxygen coupled to the element constituting the facet of the compound semiconductor layer before formation of the passivation layer and preferably the oxygen immigrated to the passivation layer from the facet of the compound semiconductor layer by plasma irradiation or heat irradiation made to the passivation layer. Oxygen contained as a structural element in the passivation layer is preferably the oxygen immigrated to the passivation layer from the. facet of the compound semiconductor layer during plasma irradiation made at times of formation of the coating layers.